The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, plaranization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either axe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened or soft surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-active agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
A reoccurring problem in CMP is the so-calledxe2x80x9cedge-effectxe2x80x9d, i.e., the tendency of the substrate edge to be polished at a different rate than the substrate center. The edge effect typically results in non-uniform polishing at the substrate perimeter, e.g., the outermost three to fifteen millimeters of a 200 millimeter (mm) wafer.
In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier structure and a first flexible membrane having a perimeter portion connected to the carrier structure. A central portion of the membrane has a lower surface that provides a substrate mounting surface, and a first volume between the first flexible membrane and the carrier structure provides a first chamber. A body located in the first chamber has a first portion that applies pressure to a first region of an upper surface of the central portion of the first flexible membrane and a second portion that is separable and movable into contact with a second region of the upper surface of the central portion of the first flexible membrane. A second chamber applies a downward load to the body to urge the second portion of the body into contact with the second region of the upper surface of the first flexible membrane.
Implementations of the invention can include one or more of the following features. The body may be annular. The second portion of the body may be bendable. A cushion may be secured to an underside of the first portion of the body to contact the upper surface of the first flexible membrane. A control ring may transmit the downward load from the second chamber to the body. The control ring may be positioned between the first flexible membrane and a retaining ring. The first flexible membrane may include a lip portion that extends inwardly from the central portion over the second portion of the body. The perimeter portion of the first flexible membrane may extends upwardly from the lip portion between the control ring and the body and outwardly over a top of the control ring. An annular spacer may be located between the second chamber and the perimeter portion of the first flexible membrane. A second flexible membrane may be secured to the carrier structure, a second volume between the second flexible membrane and the carrier structure forming the second chamber. The first volume may be located between the first flexible membrane and the second flexible membrane. The body may include a cylindrical portion extending between the first flexible membrane and the second flexible membrane.
In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier structure and a first flexible membrane having a perimeter portion secured to the carrier structure. The flexible membrane also has central portion with a lower surface that provides a substrate mounting surface. A first volume between the first flexible membrane and the carrier structure provides a first chamber. A spacer is located in the first chamber. The spacer has a portion that contacts an upper surface of the central portion of the first flexible membrane. A second chamber generates a downward load on a connecting portion of the first flexible membrane between the central portion and the perimeter portion. A connector portion of the first flexible membrane is separable and movable into contact with a top surface of the spacer.
Implementations of the invention may include one or more of the following features. Below a first pressure in the second chamber, the connector portion of the first flexible membrane may not contact the spacer. The second chamber may press an edge of the central portion of the flexible membrane against the substrate to generate a first region of increased pressure on the substrate. Above the first pressure in the second chamber, the connector portion of the first flexible membrane may contacts the spacer. The second chamber may press the spacer against the top surface of the central portion of the first flexible membrane to generate a second region of increased pressure on the substrate. Above a second pressure in the second chamber, the connector portion of the first flexible membrane may contact a top surface of the central portion of the flexible membrane. The second chamber may press the connector portion against the top surface of the central portion to generate a third region of increased pressure on the substrate. A control ring may transmit the load from the second chamber to the connector portion of the first flexible membrane. The connector portion may extend inwardly over the central portion of the flexible membrane. The control ring may rest on the lip portion of the flexible membrane. A cushion may be secured to an underside of the first portion of the spacer. The control ring may be positioned between the first flexible membrane and the retaining ring. A second flexible membrane may be secured to the carrier structure, and a second volume between the second flexible membrane and the carrier structure may form the second chamber. The first volume may be located between the first flexible membrane and the second flexible membrane. The spacer may include a cylindrical portion extending between the first flexible membrane and the second flexible membrane.
In another aspect, the invention is directed to a method for chemical mechanical polishing a substrate. In the method, a substrate is held against a polishing pad with a carrier head. A first downward load is applied to the substrate with a first chamber in the carrier head. A second downward load is generated with a second chamber in the carrier head. A first portion of the second downward load is distributed to a first area on the substrate. If the second downward load exceeds a threshold load, a second portion of the second downward load is distributed to a second area on the substrate. Relative motion is created between the substrate and the polishing pad.
Implementations of the invention may include one or more of the following features. The first and second areas may be annular.
Potential advantages of implementations of the invention may include zero or more of the following. The distribution of pressure at the substrate edge may be controlled. Both the pressure and the loading area of the flexible membrane against the substrate may be varied to compensate for non-uniform polishing. Non-uniform polishing of the substrate is reduced, and the resulting flatness and finish of the substrate are improved.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.